Telescopic booms are known in which a pulley is rotatably mounted on a pivot pin and engages a boom extension chain. The pivot pin has ends which project beyond the pulley and are diametrically threaded and received threaded portion of adjustment bolts. In order to adjust the chain following wear so as to resynchronize the relative positions of the first, second and third booms of the telescopic boom, the bolts are tightened thereby drawing the pulley pivot pin longitudinally relative to the boom, and hence moving the pulley longitudinally relative to the boom from a first fixed position (the non-synchronized position) to a second fixed position (the synchronized position).
However, since the bolts are threaded into a threaded hole in the pivot pin, then the pivot pin must be of a diameter sufficiently larger than the diameter of the bolts in order to ensure the threaded hole in the pin is sufficiently strong to withstand the loads applied to it during use. This requires a relatively large diameter pin, which in turn requires a relatively large diameter pulley. As the pulley diameter increases then the visibility of the load at the end of the boom as viewed by the machine operator may become impaired.